Solid tantalum capacitors were introduced in the 1950's, and since that time such devices have replaced many of the liquid electrolyte-containing aluminum electrolytic capacitors of similar rating used in the fabrication of electronic circuits. Solid tantalum capacitors have higher capacitance per unit volume, lower equivalent series resistance, lower temperature dependence of capacitance and equivalent series resistance, and higher reliability than the liquid electrolyte aluminum capacitors.
The high capacitance per unit volume of solid tantalum capacitors is a function of the high surface area tantalum powder used to fabricate the powder metallurgy compacts making up the anodes of electronic devices and is also a function of the high dielectric constant of the anodic oxide dielectric film. The high reliability of solid tantalum capacitors is a function of the high stability of the anodic tantalum oxide dielectric layer applied to each sintered powder metallurgy tantalum compact via an anodizing process step. The low equivalent series resistance and small temperature dependence of capacitance and equivalent series resistance are largely a function of the manganese dioxide cathode material used in the fabrication of these devices.
The manganese dioxide cathode material in solid tantalum capacitors is produced in situ via pyrolysis of manganese nitrate solution introduced into the powder metallurgy anode bodies by a dipping step prior to the pyrolysis step. The manganese nitrate dipping and pyrolysis sequence is repeated until the pore structure is sufficiently coated with manganese dioxide.
After the application of the manganese dioxide cathode material to the sintered and anodized powder metallurgy tantalum anode compacts is complete, the compacts are coated with carbon and silver paint, then assembled into finished devices. The finished devices may be on the leaded (hermetically-sealed metal can, molded, or fluidized bed epoxy coated) or surface mount (molded or conformally resin coated) configuration.
Manganese dioxide is a complex substance having many crystal forms, hydration states, and crystal densities. In addition to the above variables, manganese dioxide produced via pyrolysis of manganese nitrate solutions is of varying porosity and surface smoothness depending upon pyrolysis conditions. The focus of a good deal of the development work conducted in the field of solid tantalum capacitors has been the production of manganese dioxide coatings which are dense, adherent, and highly electrically conductive.
Early in the development of solid tantalum capacitors, it was recognized that carrying out the pyrolysis process in the presence of steam gives rise to smoother, denser, and more electrically conductive manganese dioxide than when the pyrolysis is carried out in air. A denser, smoother, and more conformal manganese dioxide coating can be obtained with pyrolysis carried out in an essentially steam atmosphere. Prior to the development of steam atmosphere pyrolysis, the manganese dioxide pyrolytic coatings on tantalum capacitors produced in air were sufficiently non-uniform to require mechanical sizing, such as by external grinding, prior to fabrication of the finished devices.
It was discovered that confining the pyrolysis reaction gases in close proximity to the manganese nitrate coated substrate gives rise to the production of manganese dioxide having higher density and conductivity than manganese dioxide produced in an atmosphere of air or steam alone (“Electrical Properties of Manganese Dioxide and Manganese Sesquioxide”, by Peter Klose, Journal of the Electrochemical Society, Vol. 117, No. 7, pages 854-858). Others made use of this effect, i.e., the improvement in manganese dioxide density and conductivity when the pyrolysis gases are confined in close proximity to the reaction mass, to produce tantalum capacitors having improved electrical parameters (lower leakage current and dissipation factor, higher capacitance) by confining the manganese nitrate solution-dipped anodes within small radiant ovens having a small degree of positive pressure, with or without horizontal circulation of the oven atmosphere. See U.S. Pat. Nos. 4,038,159, 4,042,420, 4,105,513, and 4,148,131; also described by Nishino, et. al., at the Manganese Dioxide Symposium, 1980, Tokyo, published by The Electrochemical Society, 1981, Symposium Proceedings, pages 305-320.
Confining decomposition gases from manganous nitrate pyrolysis (mainly nitrogen dioxide and steam) in close proximity to manganese nitrate solution-dipped anodes in order to obtain improved pyrolytic manganese dioxide properties has several drawbacks under manufacturing conditions. In order to obtain uniform results, the pyrolysis oven must be loaded with the same number of anodes of the same size containing the same amount of the same concentration of manganous nitrate. However, it is very desirable to be able to vary the number and size of the anodes undergoing pyrolysis in order to meet manufacturing demands.
Aronson, et. al., U.S. Pat. No. 4,164,455, reasoned, because the major nitrogen-containing species evolved during manganese nitrate pyrolysis is nitrogen dioxide, that this is the material responsible for the results obtained in Klose's experiments and Nishino's pyrolysis process. Aronson found similar results could be obtained by employing a small-volume oven into which is introduced a stream of nitrogen dioxide as well as steam. The introduction of nitrogen dioxide as well as into the oven would seem to free the process from a dependence upon loading uniformity from pyrolysis run to pyrolysis run in order to obtain uniform pyrolytic manganese dioxide properties.
A series of experiments indicated that gaseous oxidizing agents more oxidizing than nitrogen dioxide, such as nitric acid, hydrogen peroxide/nitric acid mixtures, and ozone, are significantly more effective than nitrogen dioxide in facilitating the production of the higher density, higher electrical conductivity beta crystal form of manganese dioxide associated with superior electrical performance in the finished solid capacitors (U.S. Pat. No. 5,622,746, and “A Process For Producing Low ESR Solid Tantalum Capacitors”, by Randy Hahn and Brian Melody, presented at The 15th Annual Capacitor and Resistor Technology Symposium, Mar. 11, 1998, Symposium Proceedings, pages 129-133). The oxidizing agent(s) may be present at relatively low concentrations, e.g. 1-2% of ozone, to 50% or more of the oven atmosphere.
The oxidizing agents employed by Hahn tend to be expensive and corrosive (nitric acid) as well as unstable at pyrolysis temperatures (hydrogen peroxide, ozone). The instability of these reagents makes frequent oven atmosphere turnover necessary in order to maintain the most favorable conditions for high density/high conductivity manganese dioxide production, while the expense of these materials mandates minimal oven size for economic process operation, i.e., a 50% reduction in oven volume for the same oven capacity, in terms of anodes processed in a batch, results in a 50% savings of oxidizing agent and steam consumed per part processed.
Oven size (volume versus anode capacity) is not the only consideration in oven design. Circulating air ovens have been found to offer several advantages over non-circulating radiant ovens for the processing of tantalum anodes through the manganese nitrate pyrolysis process. Circulating air (circulating atmosphere) ovens are more readily maintained at uniform temperature than non-circulating ovens. Circulating air ovens heat the anodes more rapidly than radiant ovens maintained at the same temperature. Atmospheric doping and composition control is more easily accomplished with a circulating atmosphere oven than with a radiant oven.
Applying manganese dioxide to tantalum powder metallurgy anodes provided a decided advantage for pyrolysis ovens having top-down air flow. Top-down air flow dries the tops of the anodes, which are suspended from bars held in a horizontal rack (process lid) faster than the lower portions of the anodes, resulting in liquid phase material being transported to the tops of the anodes by capillary action, counterbalancing the tendency for the liquid manganese nitrate solution to migrate to the lower portions of the anodes due to the action of gravity. The overall result is the production of more uniform manganese dioxide coatings in top-down circulating air pyrolysis ovens.
In order to direct the airflow inside of circulating air process ovens, conventional ovens contain ducts, baffles, and plenums through which the oven atmosphere flows under the impetus of a motorized fan or fans contained within the ductwork. One of the most difficult goals to accomplish in circulating atmosphere oven design is the production of uniform and laminar flow of the oven atmosphere past the objects to be heated, which are contained within the main chamber of the oven during use. In order to render the atmospheric flow uniform across the entire load within an oven, oven manufacturers employ expensive plenums, stacked diffusion screens, and multiple blowers in oven construction. One consequence of using extensive plenums and diffusion screen stacks, etc., is that the volume of the oven atmosphere is many times larger than the volume of the parts being processed. The resulting large size and cost of circulating atmosphere ovens are disadvantageous for the user of these devices. The large volume, associated with the ducting and plenums employed in conventionally designed ovens, also necessitates the use of a relatively large amount of atmospheric doping chemicals for applications such as the manufacture of tantalum capacitors.
What is desired, then, is a circulating process oven designed and fabricated so as to facilitate laminar and uniform atmospheric flow within the oven without the need for the large volume of ducting, plenums, and diffusion screens required to produce uniform oven atmosphere circulation in ovens atmosphere circulation in ovens of conventional design in order to minimize the parasitic oven volume such as ducting, plenums, diffusion screens, etc. versus the useful oven volume in which the load resides during processing.